Uses and dangers of oxygen therapy : report of a sub-committee of the Standing Medical Advisory Committee.

  • Scotland. Standing Medical Advisory Committee.
Date:
1969
Catalogue details

Licence: Public Domain Mark

Credit: Uses and dangers of oxygen therapy : report of a sub-committee of the Standing Medical Advisory Committee. Source: Wellcome Collection.

    13/112 (page 5)
    Important points to note are: Oxygen: The blood approaches near saturation (about 97% of maximum amount carried) at 90 mm Hg. This is approximately the normal pressure of oxygen in a healthy lung. Further rises of oxygen tension, which would occur with extreme hyperventilation or oxygen therapy only causes a very slight increase of the oxygen carried. The blood carries well over 80% of its total capacity for oxygen at as low an oxygen tension as 50 mm Hg. which is nearly half the normal lung and arterial blood gas tension. At lower oxygen tensions, the amount carried falls rapidly. These properties give a considerable safety margin in the uptake of oxygen and facilitate its liberation to the tissues of the body. In severe respiratory failure the oxygen tension in the blood leaving the lungs and entering the arteries is usually 50 mm Hg. or below. If it continues to fall, then dangerously low amounts of oxygen are carried in the blood. Controlled oxygen therapy, say 30%, raises the oxygen tension only moderately (55 to 80 mm Hg.) but this moderate increase causes a very marked rise in the oxygen content. Thus grave hypoxaemia (inadequate oxygen in the blood) is relieved, but undesirably high or abnormally high levels of oxygenation are avoided. Carbon dioxide: It will be seen from the diagram that the blood has a far greater capacity for carbon dioxide than oxygen at any particular pressure (approximately three times greater). Further, the carbon dioxide content of the blood rises or falls in a manner approximately proportionate to the tension of carbon dioxide over the whole range here illustrated. Con- sequently the carbon dioxide content of the blood leaving the whole or part of the lungs falls or rises markedly with over or underyentilation of the lung, responding faithfully to changes in lung carbon dioxide tension. This is in contrast to the oxygen content of the blood leaving the lung which, although falling markedly with underventilation of lung (low oxygen tension), is little affected by overventilation or oxygen therapy which cause a rise of oxygen tension from normal to supernormal levels. 15. The efficiency with which the lungs perform their function of adding oxygen to the blood can be assessed by measuring the partial pressure of oxygen in the blood leaving the lungs; this is the blood which enters the left heart and then flows in the systemic arteries. It can be sampled by arterial puncture, a simple and safe procedure in every day use in investigation of chest diseases in hospital patients. An arterial PO, below 70 mm Hg., when a patient is breathing air at sea-level, is abnormal. There are four mechanisms which may result in a fall in the arterial PO below these normal levels, in a patient breathing air at sea level. Ventilatory Failure 16. The patient may not be ventilating his lungs with a sufficient amount of fresh air. This results in a reduced elimination of carbon dioxide in addition to a deficiency of oxygenation, so that the arterial PCO, is increased above normal (35-45 mm Hg.) and the arterial PO, reduced below normal. This situation is known as ventilatory failure; it can arise suddenly from acute obstruction of the airway, in drowning, electrocution, or from overdosage with narcotic drugs, or from poisoning by agents producing muscular paralysis. Ventilatory failure can also develop acutely in the course of poliomyelitis, polyneuropathy, in acute diseases of muscle, myasthenia gravis, and as a complication of crushed chest injuries. The most common type of ventilatory failure in clinical practice in Britain arises as a complication of chronic bron- chitis and emphysema. In these patients an exacerbation of a pre-existing chest infection can easily precipitate profound hypoxia and carbon dioxide retention. Indeed many of these patients with chronic bronchitis develop a state of chronic ventilatory failure, which although punctuated by exacer- bations, may continue for many years, even though ultimately fatal. The recognition of ventilatory failure carries important implications for oxygen therapy and this is discussed in more detail in Chapter 3,